Optical films comprising polymeric resin films are used in a variety of electronic display applications. In particular, resin films are used as protective cover sheets for light polarizers in Liquid Crystal Displays (LCD).
LCDs may contain a number of optical elements that may be formed from resin films. The structure of reflective LCD's may include a liquid crystal cell, one or more polarizer plates, and one or more light management films. Liquid crystal cells are formed by dispersing liquid crystals such as twisted nematic (TN) or super twisted nematic (STN) materials between two electrode substrates.
Polarizer plates (also commonly called “polarizers”) are widely used in displays to control incoming and outgoing light. For example, in LCDs, a liquid crystal cell is typically situated between a pair of polarizer plates. Incident light polarized by the first polarizer plate passes through a liquid crystal cell and is affected by the molecular orientation of the liquid crystal, which can be altered by the application of a voltage across the cell. The altered light goes into the second polarizer plate. By employing this principle, the transmission of light from an external source, including ambient light, can be controlled. LCD's are quickly taking over from traditional CRTs in computer monitors and TVs because of their lower energy consumption and thinness.
Polarizer plates are typically a multi-layer element comprised of a polarizing film sandwiched between two protective cover sheets. Polarizing films are normally prepared from a transparent and highly uniform amorphous resin film that is subsequently stretched to orient the polymer molecules and stained with iodine or organic dichroic dyes to produce a dichroic film. An example of a suitable resin for the formation of polarizer films is fully hydrolyzed polyvinyl alcohol (PVA). Because the stretched PVA films used to form polarizers are very fragile and dimensionally unstable, protective cover sheets are normally laminated to both sides of the PVA film to offer both support and abrasion resistance. Protective cover sheets of polarizer plates are required to have high uniformity, good dimensional and chemical stability, and high transparency. After formation of the polarizing film, a protective cover sheet is laminated to each side of the polarizing film using glue or adhesive.
Originally, protective cover sheets were formed from glass. Today, protective cover sheets are generally multi-layer polymeric films that comprise a low birefringence polymer film and one or more other optical films that serve specific functions such as an antiglare layer, hardcoat layer, low reflection layer, antireflection layer, and compensation layer, for example. Generally, these antiglare, hardcoat, low reflection, antireflection, and compensation layers are applied in a process step that is separate from the manufacture of the low birefringence polymer film.
Although many polymers have been suggested for use as the low birefringence polymer film in protective cover sheets, cellulosics, acrylics, cyclic olefin polymers, polycarbonates, and sulfones are most commonly used. Polymers of the acetyl cellulose type are commercially available in a variety of molecular weights as well as the degree of acyl substitution of the hydroxyl groups on the cellulose backbone. Of these, the fully substituted polymer, triacetyl cellulose (TAC) is commonly used to manufacture low birefringence polymer films for use in protective cover sheets for polarizer plates.
Prior to lamination to the PVA polarizing film the cover sheet normally requires a surface treatment to insure good adhesion to the PVA film. When TAC is used in the protective cover film of a polarizer plate, the TAC film is subjected to treatment in an alkali bath to saponify the TAC surface to provide suitable adhesion to the PVA film. The alkali treatment uses an aqueous solution containing a hydroxide of an alkali metal, such as sodium hydroxide or potassium hydroxide. After alkali treatment, the cellulose acetate film is typically washed with a weak acid solution followed by rinsing with water and drying. This saponification process is both messy and time consuming. U.S. Pat. No. 2,362,580 describes a laminar structure wherein two cellulose ester films each having a surface layer containing cellulose nitrate and a modified PVA is adhered to both sides of a PVA film. JP 06094915A discloses a protective film for polarizer plates wherein the protective film has a hydrophilic layer which provides adhesion to PVA film. U.S. Patent Application 2003/0002154 describes an optical stack that includes an intrinsic polarizer. Optically functional coating are disposed on one or both of the surfaces of the intrinsic polarizer.
Some LCD devices may contain a protective cover sheet that also serves as a compensation film to improve the viewing angle of an image. Compensation films (i.e. retardation films or phase difference films) are normally prepared from amorphous films that have a controlled level of birefringence either by uniaxial stretching or by coating with discotic dyes.
There is a need for a new process to manufacture polarizer plates that eliminates the need for the saponification and lamination processes normally employed. There is also a need to provide a process that eliminates the necessity to apply additional layers such as an antiglare layer, hard coat layer, antireflection layer, compensation layer, etc. onto the low birefringence polymer film in a separate manufacturing operation. Additionally, there is a need to provide thinner polarizer plates for today's thin and light weight display devices, however, thin optical films are difficult to handle without wrinkling during a conventional lamination process. Finally, optical films such as those used as conventional protective cover sheets for polarizer plates are susceptible to scratch and abrasion, as well as the accumulation of dirt and dust, during their manufacture, shipping/handling, and lamination to a polarizing film.